Am I retarded/ mentally challenged?

When I talk to people, and say/do something dumb, they automatically call me retarded or mentally challenged. It hurts me so bad, because I get called this on a regular bases. Am I retarded for joking around and making a goon out of myself? I get pretty good grades in school, and i'm not really slow, so I don't think myself as retarded. but it seems like everyone else does. Am I really retarded?